This invention relates generally to portable cellular phones and more particularly to a dual mode phone that can transmit and receive both analog and digital telecommunication data.
Many cellular telephones are designed to receive and transmit both analog and digital signals. FIG. 1 is a prior art schematic diagram showing typical circuitry for performing both analog and digital telephonic communications.
When transmitting digital data in a typical cellular telephone, in phase (I) and quadrature phase (Q) signals are fed from a digital signal processor (DSP) 10 into an IQ modulator 12. The I and Q signals modulate a local oscillator signal (LO) 21. The modulated I and Q signals are then passed through a power amplifier 16 and transmitted from an antenna 20.
For analog transmissions, a frequency modulated (FM) voice signal is output from an analog FM circuit 22 and modulates the LO. The modulated analog signal is amplified by power amplifier 16 and then transmitted from antenna 20. Thus, during transmission of digital data the analog transmitter circuitry (22, 26) is not used and during transmission of analog data the digital transmitter circuitry (12) is not used.
For receiving signals, the antenna 20 receives radio frequency (RF) signals either encoded in analog or digital formats. The RF signal from antenna 20 is mixed down to a lower intermediate frequency (IF) at mixer 30. For analog signals, a discriminator 34 extracts the FM signal and feeds the result back to the FM analog circuit 22.
The discriminator 34 extracts a baseband signal from the received FM input. The amplitude (A) and phase (.phi.) of the RF signal are then extracted from the baseband signal. The I and Q signals are then generated according to the relationship, Acos.phi.+jAsin.phi., wherein I=Acos.phi. and Q=Asin.phi.. Alternatively, a separate IQ demodulator 32 is used to process digital RF data.
The system shown in FIG. 1 utilizes separate circuitry for transmitting and receiving digital and analog data. For example, in the transmit portion of the cellular phone, the IQ modulator 12 is used for modulating digital data and the FM modulator 26 are used for modulating analog data. On the receiver side of the dual mode cellular phone, the discriminator 34 is used for demodulating analog data and additional circuitry is utilized to translate the discriminator data into a digital format. Alternatively, a separate IQ demodulator 32 is used to demodulate digital signals.
U.S. Pat. No. 5,228,074 to Mizikovsky shows a dual mode analog and digital phone that uses separate DQPSK demodulator circuitry 29 for decoding digital voice signals. Analog voice signals are received and demodulated by a separate FM demodulator 18. Multiple switches are moved back and forth depending upon the transmit and receive modes of the cellular phone (i.e., analog or digital). The separate circuitry used in Mizikovsky and in FIG. 1 for processing both analog and digital data increase phone complexity and cost while reducing system reliability.
Another problem with the standard FM modulation and demodulation schemes described above is that precise frequency tuning is required to accurately process the voice signals at the correct modulation frequencies and gain values. Small variations or inconsistencies in signal tuning distort the sound quality of transmitted and received voice signals. Thus, it would be desirable to utilize as much digital circuitry as possible to process voice signals.
Dual mode phone systems must operate according to industry standard IS-54B for 800 MegaHertz (MHz) signal transmissions. Complex phone circuitry and large power sources are required to implement the IS-54B standard. However, consumer demand requires that cellular phones be inexpensive, small, light weight and operate for long periods of time on relatively small battery packs.
Accordingly, a need remains for a simple low cost dual mode analog and digital cellular telephone that is inexpensive and can operate on a small portable power supply for long periods of time.